In a typical cellular wireless communication system, a radio access network (RAN) includes a plurality of base stations, each of which radiates to define one or more air interface coverage areas such as a cell and cell sectors in which wireless communication devices (WCDs) can be served by the RAN. Generally or on a region by region basis, the base stations of the system may then all be connected with common RAN infrastructure, such as a base station controller (BSC), radio network controller (RNC), mobile switching center (MSC), packet data serving node (PDSN), or the like, which may function to control certain aspects of base station operation and certain aspects of the air interface and may provide connectivity with one or more transport networks such as the public switched telephone network (PSTN) and the Internet for instance. With this arrangement, a WCD served by the RAN may wirelessly communicate with other served WCDs or with entities on the one or more transport networks, with communications passing through the air interface, one or more base stations, and the RAN infrastructure.
The base stations in such a system may communicate with served WCDs according to an agreed air interface protocol, examples of which include CDMA (code division multiple access), LTE (long term evolution), WiMAX (wireless interoperability for microwave access), GSM (global system for mobile communications), and others now known or later developed. As a general matter, communications in the direction from the RAN to WCDs define a “forward link” (or downlink), while those in the direction from WCDs to the RAN define a “reverse link” (or uplink).
In practice, air interface communications in each coverage area of such a system may occur on one or more carrier frequencies. Further, the air interface may be divided into channels through a mechanism such as time division multiplexing, code division multiplexing, and/or frequency division multiplexing, for instance. For instance, the forward link may define a pilot channel on which the base station broadcasts a pilot signal for use by WCDs to detect and evaluate coverage, and one or more other control and traffic channels on which the base station may transmit control data and bearer data WCDs. Further, the reverse link may likewise define one or more control and traffic channels on which the WCDs may transmit control data and bearer data to the base station.
Control data that passes over the air interface between a base station and a WCD may function in some manner to control or facilitate operation of the wireless communication system. By way of example, the control data may include power-control signaling for managing the power of air interface transmissions. Further, the control data may include various overhead messages regarding operational parameters of the coverage area or adjacent coverage areas, such as system parameter messages, channel list messages, and neighbor list messages for instance. Depending on the network configuration, the RAN infrastructure may be involved with some or all of this control communication. Therefore, some or all of this control data, or derivations of this control data, may ultimately pass over the backhaul link between the base station and the RAN infrastructure, possibly along with other control data.
The bearer data that passes over the air interface between a base station and a WCD, on the other hand, is typically the application-layer communication content, such as voice data, text-message data, browsing data, or real-time media-session data for instance. As the RAN infrastructure may provide connectivity with one or more transport networks over which such bearer data or derivations of such bearer data may travel, certain bearer data may similarly pass over the backhaul link between the base station and the RAN infrastructure.
In general, when a WCD is actively engaged in a call (e.g., voice call or data session) in a coverage area of the RAN, the WCD may regularly monitor the strength of the pilot signal broadcast by the RAN in that coverage area and the strength of pilot signals broadcast by the RAN in other coverage areas extending to the WCD's position. If the WCD detects a pilot signal from another coverage area that is sufficiently stronger than the pilot signal of the WCD's current coverage area, the WCD may transmit a signal to the RAN to request a handoff of the call to the detected coverage area. The RAN may then assign traffic channel resources to the WCD for use in the new coverage area and direct the WCD to continue the call in the new coverage area.
Under some air interface protocols, a WCD may also be capable of engaging in a call actively in multiple coverage areas at once. In such an arrangement, the WCD may have an “active set” of coverage areas in which the WCD simultaneously exchanges data, including both bearer data and control data, with the RAN. Advantageously, this active-set arrangement can help improve the quality of bearer data communications, as the WCD may exchange the same bearer data concurrently with each of its serving base stations, and both the WCD and the RAN infrastructure may then combine received copies of the bearer data together or select the best copy of received bearer data on a frame-by-frame basis for instance. Likewise, the active set arrangement may help improve control communication as well, by helping to ensure that control data reaches its destination.
In terms of power-control signaling, in the active-set arrangement, separate power control signaling could be carried out for each active set coverage areas, but aggregate changes may be made across the active set coverage areas. For instance, to control power on the forward link, the WCD may evaluate quality or strength of received signals in each active set coverage area and, (i) if the quality or strength in any of the active set coverage areas is threshold good enough, the WCD may send a power-down command on the reverse link in each of its active set coverage areas, but (ii) if the quality or strength in all of the active set coverage areas is threshold poor, the WCD may send a power-up command in each of its active set coverage areas. Further, similar processing could occur at the RAN to control power on the reverse link.